The invention relates to a method and a measuring apparatus for determining physical properties and quantities relevant to combustion of gas and gas mixtures. Physical gas properties mean in particular the density, thermal conductivity, heat capacity and viscosity as well as correlatable quantities relevant to combustion, such as the energy content, calorific value, Wobbe index, methane number and/or air requirement of the gas or gas mixture.
In gas-fuel firing control systems it is important to keep the load in the burner constant even at changing fuel gas qualities. The Wobbe index, formed from the calorific value and the root of the density ratio between air and this gas, is the appropriate index for displaying the interchangeability of gases. An identical Wobbe index will then result in a constant thermal load in the burner.
When regulating (natural) gas motors, knowledge of the calorific value at varying (natural) gas qualities is necessary to achieve an increase of performance or efficiency, while for gas the methane number—by analogy to the octane number for gasoline—is used to assess ignition behaviour (knocking effect or misfiring).
An optimal combustion process requires a correct mixing ratio between fuel gas and air-, known as “air requirement”. Soot (flue gas) usually forms if there is too little air, and this may damage fuel cells in particular. Too much air during combustion results in reduced performance. The optimal value depends on the application concerned, but changes again with varying gas qualities.
Correlation methods for calculating quantities relevant to combustion have been described in academic literature, see for example U. Wernekinck, “Gasmessung and Gasabrechnung” (Gas metering and gas billing), Vulkan publishers, 2009, ISBN 978-3-8027-5620-7. The following combinations of measured variables are used in this connection:
A. Dielectric constant, sonic velocity, CO2 content
B. Sonic velocity at 2 pressures, CO2 content
C. Thermal conductivity at 2 temperatures, sonic velocity
D. Thermal conductivity, heat capacity, dynamic viscosity
E. Thermal conductivity, infrared absorption (not dispersive)
F. Infrared absorption (dispersive)
There are currently only a few commercially available devices that are approved for calorific value readings, e.g. the EMC500 device by RMG-Honeywell (Type D plus CO2 content) or the Gas-lab Q1 device by Elster-Instromet (Type E plus CO2 content). However, due to the high acquisition costs, none of these devices is suitable for mass distribution.
Integrated CMOS hot-wire anemometers are able to take a microthermal measurement of thermal conductivity as well as of mass flow. For this technology, reference is made to the publication of D. Matter, B. Kramer, T. Kleiner, B. Sabbattini, T. Suter, “Mikroelektronischer Haushaltsgaszähler mit neuer Technologie” (Micro-electronic household gas meter using new technologies), published in Technisches Messen 71, 3 (2004), pp. 137-146. It differs from conventional thermal mass flow meters by taking the measurement directly in the gas flow and not from the outside on a metal capillary tube that encompasses the gas flow.
EP 2 015 056 A1 describes a thermal flow sensor for determining a quantity relevant to combustion, based on a thermal conductivity reading if the mass flow is basically known. A critical nozzle is used to keep the mass flow constant, and the aim is to correct the gas type dependence of the critical nozzle by means of the thermal conductivity. However, the information on the correlation of quantities relevant to combustion is limited to two more or less independent measured variables and thus does not permit validation of the measured data.
WO 2004/036209 A1 describes a sensor for determining a quantity relevant to combustion where the mass flow is kept constant and where a value that is proportional to the heat capacity is identified by means of a thermal measurement. Since the described sensor is not a microthermal sensor, it is not possible to draw conclusions regarding thermal conductivity; this means that the determination of the heat capacity and the quantities relevant to combustion derived therefrom is only possible up to one proportionality factor. As a result, an additional calibration with known gas compositions is required. In addition, the information on thermal conductivity, and thus the means to correlate thermal conductivity λ with a quantity relevant to combustion is omitted. Furthermore, the accuracy of this method is limited by the occurring variations of the inaccessible thermal conductivity λ.